How does the lac operon work?

The lac operon has 3 components: the promoter (Binding site for RNA polymerase), the operator (binding site for the repressor) and the structural genes that code for the enzymes required for lactose metabolism. The gene coding for the repressor protein is also located nearby, usually a little before the operon. When lactose is absent, the repressor protein binds to the operator and prevents the RNA polymerase from binding to the promoter. As a result, the enzyme genes cannot be transcribed and the enzymes are not produced. When lactose is present and the enzymes are needed, lactose diffuses into the cell and binds to the repressor protein. This changes the conformation (3D shape) of the repressor. As its shape is different, it can no longer bind to the operator. The RNA polymerase can thus bind to the promoter and the enzyme genes are transcribed and translated.